We know much about HIV transcription and replication. During the past two decades, the roles played by basal transcription complexes, NF-kB and Tat in initiation and elongation of viral transcription have been placed on solid scientific foundation. In addition, the co-activator of NF-kB and the viral transactivator Tat, the positive transcription elongation factor b (P-TEFb), has been demonstrated to phosphorylate the C-terminal domain (CTD) of RNA polymerase II (RNAPII), Spt5 from DSIF and RD from NELF, thus allowing transcription complexes to elongate. P-TEFb itself is under exquisite control in cells, which determines their state of differentiation and proliferation. At least three additional cyclin dependent kinases play a critical role in co- transcriptional processing of HIV RNA, namely Cdk11, 12 and 13, some of which affect 3'end formation (Cdk11) and others the complex splicing of viral transcripts (Cdk12/Cdk13). Recently, we found that the latter two bind CycK, which was thought previously to be another component of P-TEFb. Importantly, CycK does not bind Cdk9, which together with CycT1 or CycT2 forms P-TEFb. This proposal will address in intricate detail how Tat interferes with the formation of inactive P-TEFb complexes in cells, how certain inhibitors of histone deacetylases do the same and how these new CycK complexes affect the splicing of HIV transcripts. In the process, we will have learned how Tat sustains optimal viral replication in infected cells, how drugs such as HMBA, SAHA and specific HDACis perturb the metabolism of P-TEFb and how these new CycK:Cdk complexes affect RNAPII as well as alternative splicing factors. New targets for inhibiting HIV replication will be revealed together with candidate compounds that should find future clinical utility. PUBLIC HEALTH RELEVANCE: Tat transactivation is essential for HIV replication. Tat binds CycT1 and Cdk9 that convert an initiating to an elongating RNA polymerase II. Next, CycK binds Cdk12 and Cdk13 that regulate the co-transcriptional processing of HIV transcripts. This proposal will investigate how Tat uses these complexes for optimal HIV replication and how these Cdks could be manipulated pharmacologically in the future.